1. Field of the Invention
The present invention relates to microstructure devices such as micromirror elements, acceleration sensors, angular-speed sensors and vibration elements made by micromachining technology.
2. Description of the Related Art
In recent years, microstructure devices manufactured by means of micromachining technology are gathering attention, and efforts are being made for making practical application of element devices which have a micro-structure. Microstructure devices include micromirror elements, acceleration sensors, angular-speed sensors and other micro moving devices which have tiny moving parts or vibrating parts therein. Micromirror elements are used in the field of optical disc technology and optical communications technology for example, as a light reflection device. Acceleration sensors and angular-speed sensors find areas of application in the field of attitude control of robots, correction of camera shake, and so on. These micro moving devices generally include a fixed structural part, a moving part relatively displaceable with respect to the fixed structural part, and a connecting part which connects the fixed structural part and the moving part each other. Microstructures as described are disclosed in the following Patent Documents 1 through 3 for example:                Patent Document 1: JP-A-2003-19700        Patent Document 2: JP-A-2004-341364        Patent Document 3: JP-A-2006-72252        
FIG. 35 and FIG. 36 show a conventional microstructure device 80 as an example. FIG. 35 is a plan view of the microstructure device 80 whereas FIG. 36 is a sectional view taken in lines XXXVI-XXXVI in FIG. 35.
The microstructure device 80 includes a first structural part 81, a second structural part 82 and a connecting part 83 which connects the first structural part 81 and the second structural part 82 with each other. When the microstructure device 80 of such a principal structure serves as a micro moving device, the first structural part 81 represents the moving part, the second structural part 82 represents the fixed structural part, and the moving part and the fixed structural part are connected with each other by the connecting part 83.
FIG. 37 and FIG. 38 show a process of forming the microstructure device 80. FIG. 37 and FIG. 38 show a section in a series to illustrate how the first structural part 81, the second structural part 82 and the connecting part 83 are formed. The section featured in the figures is a conceptual composite collected from a plurality of fragmentary sections of a row material substrate (wafer) to which a series of manufacturing operations are made.
In the manufacture of the microstructure device 80, first, a material substrate 90 as shown in FIG. 37(a) is prepared. The material substrate 90 is an SOI (Silicon on Insulator) wafer, and has a laminated structure including a silicon layer 91, a silicon layer 92 and an insulation layer 93 between the silicon layers. The insulation layer 93 has a thickness of about 1 μm.
Next, as shown in FIG. 37(b), anisotropic dry etching is performed to the silicon layer 91 via a predetermined mask, to form structures to be built on the silicon layer 91 (i.e. the first structural part 81, part of the second structural part 82, and the connecting part 83). In this step, a predetermined etching apparatus equipped with a vacuum chamber is used to perform the dry etching in the vacuum chamber under predetermined vacuum conditions.
Next, as shown in FIG. 37(c) and FIG. 37(d), a sub-carrier 94 is bonded onto the silicon layer 91 side of the material substrate 90 via the bonding member 95. The bonding member 95 is provided by resist, grease or sealant for example. In this step, the material substrate 90 and the sub-carrier 94 are bonded together under heat and pressure. A purpose of bonding the sub-carrier 94 in such a way is to prevent damage to the material substrate 90 and to the etching apparatus in the next manufacturing step. In the next manufacturing step, etching is performed to the silicon layer 92 in the vacuum chamber of the etching apparatus. In this process, mechanical strength of the material substrate 90 decreases substantially because of the etching performed to the silicon layer 92, with the silicon layer 91 having already been etched. The sub-carrier 94 serves as a reinforcing member for the material substrate 90, and prevents the material substrate 90 from breaking. If the material substrate 90 breaks in the vacuum chamber, broken pieces can damage the etching apparatus. Therefore, damage prevention of the material substrate 90 by the sub-carrier 94 also contributes to damage prevention of the apparatus.
In the manufacture of the microstructure device 80, next, as shown in FIG. 38(a), anisotropic dry etching is performed to the silicon layer 92 via a predetermined mask, to form a structure to be built on the silicon layer 92 (i.e. part of the second structural part 82). Like the step described above with reference to FIG. 37(b), this step also employs a predetermined etching apparatus equipped with a vacuum chamber to perform the dry etching in the vacuum chamber under predetermined vacuum conditions.
Next, as shown in FIG. 38(b), the material substrate 90 is separated from the sub-carrier 94. This step is performed outside of the vacuum chamber of the etching apparatus. Thereafter, as shown in FIG. 38(c), the insulation layer 93 is subjected to isotropic etching in order to remove exposed portions of the insulation layer 93. Through the above-described process, the microstructure device 80 is completed.
However, in the process of making the microstructure device 80, the connecting part 83 is likely to be broken after the step described with reference to FIG. 38(a).
In the step described with reference to FIG. 37(b), the silicon layer 91 is partially etched off, exposing part of the insulation layer 93 in the silicon layer 91. Then, as the step in FIG. 38(a) proceeds or finishes, the insulation layer 93 is also exposed in the silicon layer 92, with portions S (double-side exposed portions) of the insulation layer 93 which are bonded neither to the silicon layer 91 nor to the silicon layer 92. The insulation layer 93 is substantially thin and the portions S are brittle enough to fracture easily.
During the step in FIG. 38(a) which is performed in the vacuum chamber, and until the separation thereafter of the material substrate 90 from the sub-carrier 94 as shown in FIG. 38(b) outside the vacuum chamber, a constant pressure (e.g. a predetermined level of vacuum) is maintained on the surface of the insulation layer 93 exposed in the silicon layer 91. On the contrary, the surface of the insulation layer 93 exposed in the silicon layer 92 is subject to pressure changes: During the step in FIG. 38(a), the surface is under a constant level of vacuum which is set for and maintained in the vacuum chamber, but the surface comes under a normal atmospheric pressure after the vacuum in the vacuum chamber is broken. As a result, the two surfaces of the portions S in the insulation layer 93 are pressed by substantially different pressures at least in one of the two occasions i.e. during the step in FIG. 38(a) and for a certain period of time thereafter. The substantial pressure difference can be a cause of the fracture in the portions S.
There is another cause if the bonding member 95 is formed of a volatile material. Under the bond between the material substrate 90 and the sub-carrier 94 in FIG. 38(a), spaces in the silicon layer 91 backed by the insulation layer 93 is filled with gas as the bonding member 95 evaporates, and as the pressure increases in the spaces, risk of fracture increases for the insulation layer 93, i.e. the portions S.
The fracture occurs anywhere in the portions S (double-side exposed portion) of the insulation layer 93. For example, once a fracture Z occurs as shown in FIG. 39(a), the fracture Z often extends as shown in FIG. 39(b) for example, within the portion S, and then further in the insulation layer 93, crossing the area of connection with the connecting part 83. When the fracture Z crosses the area of the insulation layer 93 connected with the connecting part 83, an impact acts upon the connecting part 83, destroying the connecting part 83.
As described, the conventional technique is faced by challenges in manufacturing microstructure devices which include the first structural part, the second structural part and the connecting part connecting the first and the second structural part.